White, oriented films made from crystallizable thermoplastics of thickness from 10 to 300 μm are well known.
These films comprise neither UV-stabilizer as a light-stabilizer nor flame retardants. For this reason, neither the films nor the items produced from them are suitable for outdoor applications where fire protection or low-flammability is required. The films do not pass the fire tests of DIN 4102 Part 2 and Part 1, nor that of UL 94.
In outdoor applications, even after a short period, the known films show yellowing and deterioration in mechanical properties, due to a photo-oxidative degradation by sunlight.
EP-A-0 620 245 describes films with improved heat resistance. These films comprise antioxidants suitable for scavenging free radicals formed in the film and for degrading any peroxide formed. However, this specification makes no proposal as to how the UV resistance of films of this type may be improved.
DE-A 2 346 787 describes a low-flammability plastic. Besides the plastic, the claims also cover the use of the plastic to give films and fibres.
When films were produced from this phospholane-modified polymer the following shortcomings were apparent:
The plastic is very susceptible to hydrolysis and has to be thoroughly pre-dried. When the plastic is dried using dryers of the prior art, the plastic cakes and a film can only be produced under very difficult conditions.
The films produced under extreme and uneconomic conditions embrittle at high temperatures, that is to say their mechanical properties decline sharply as a result of rapid embrittlement, making the film unusable industrially. This embrittlement arises after as little as 48 hours at high temperatures.
The object of the present invention was to provide a white, flame retardant, UV resistant, oriented film having a thickness of from 10 to 300 μm, which can be produced cost-effectively, can readily be oriented and has good mechanical and optical properties and above all is flame retardant and does not embrittle at high temperatures and has a high UV resistance.
Flame retardancy means that the white film comprises with the conditions of DIN 4102 Part 2 and in particular the conditions of DIN 4102 Part 1 in tests known as fire protection tests, and can be assigned to construction materials class B2 and in particular B1 for low-flammability materials.
The film should also pass the UL 94 test known as the “Vertical Burning Test for Flammability of Plastic Material”, so that it can be placed in class 94 VTM-O. This means that the film is no longer burning 10 seconds after removal of a Bunsen burner and that after 30 seconds there is no smouldering, and also that there are no drops of burning material during the entire period of the test.
High UV-resistance means that the films suffer no damage or only extremely little damage when exposed to sunlight or other UV-radiation, and therefore that the films are suitable for outdoor applications and/or critical indoor applications. In particular when the films are used outdoors for a number of years they should in particular not yellow, nor become brittle and not have surface-cracking, and also have unimpaired mechanical properties. High UV-resistance therefore implies that the film absorbs the UV light and does not transmit light until the visible region has been reached.
Good optical properties mean for example a homogeneous coloring, high surface gloss (>15), low luminous transmittance (<70%) and unmodified yellowing compared to films supplied with flame protection and UV.
Good mechanical properties include, inter alia, a high modulus of elasticity (EMD>3200 N/mm2; ETD>3500 N/mm2), and also good tear strengths (in MD>100 N/mm2; in TD>130 N/mm2).
Good orientability includes excellent capabilities of the film for orientation during its production, both longitudinally and transversely, without break-offs.
Cost-effective production conditions include the capability of the plastic and of any other raw material components required for producing the low-flammability film to be dried with industrial dryers which comply with the prior art like vacuum dryers, fluidized bed dryers, or fixed-bed dryers (tower-dryers). It is important that these raw materials do not cake or undergo thermal degradation. These dryers operate at temperatures of from 100° to 170° C., at which the flame retardant plastics cake inside the dryer or extruder and the carbonized mass has to be removed, making film production impossible.
In the vacuum dryer, which has the most gentle drying action, the plastic passes through a range of temperatures of from about 30° C. to 130° C. at a pressure of 50 mbar. A process known as post-drying is then required, in a hopper at temperatures of from 100–130° C. with a residence time of from 3 to 6 hours. Even in this process, flame retardant plastics according to the prior art cake to an extreme extend.
For the purposes of the present invention, no embrittlement on exposure to high temperatures for a short period means that after 100 hours of heat treatment at 100° C. in a circulating-air drying cabinet the film has not embrittled and does not have poor mechanical properties.